SELECT
CASE
WHEN s.MiddleName IS NULL THEN
   CONCAT(s.FirstName, ' ', s.LastName)
   ELSE
	CONCAT(s.FirstName, ' ', s.MiddleName, ' ', s.LastName)
END AS FullName
FROM Students s
LEFT JOIN StudentsSubjects ss ON s.Id = ss.StudentId
WHERE ss.SubjectId IS NULL
ORDER BY FullName